Add test verifying that protoc accepts trailing backslashes on Windows, which some users claim doesn't work. Seems to work on my machine.

src/google/protobuf/compiler/command_line_interface_unittest.cc

@@ -539,6 +539,24 @@ TEST_F(CommandLineInterfaceTest, WindowsOutputPathAndParameter) {
   EXPECT_EQ("bar", generator->parameter_);
 }
 
+TEST_F(CommandLineInterfaceTest, TrailingBackslash) {
+  // Test that the directories can end in backslashes.  Some users claim this
+  // doesn't work on their system.
+
+  RegisterGenerator("test_generator", "--test_out",
+                    "output.test", "Test output.");
+
+  CreateTempFile("foo.proto",
+    "syntax = \"proto2\";\n"
+    "message Foo {}\n");
+
+  Run("protocol_compiler --test_out=$tmpdir\\ "
+      "--proto_path=$tmpdir\\ foo.proto");
+
+  ExpectNoErrors();
+  ExpectGenerated("test_generator", "", "foo.proto", "Foo", "output.test");
+}
+
 #endif  // defined(_WIN32) || defined(__CYGWIN__)
 
 TEST_F(CommandLineInterfaceTest, PathLookup) {